Require "enum-compat" instead of "enum34"

[staticglobal]

The enum34 package is only necessary for Python < 3.4, yet this package states it as a hard requirement, which is used no matter which version of Python is running. This might not be a big deal except enum34 fails to install on newer versions of python when being built from source

$ pip install --user --no-binary :all: pykmip
Collecting pykmip
  Using cached https://files.pythonhosted.org/packages/48/2d/2a18894afb0666f6b273814e5e89460bfb63d0569ee35bcf4006505f39df/PyKMIP-0.8.0.tar.gz
Requirement already satisfied: cryptography in /usr/lib64/python3.6/site-packages (from pykmip)
Collecting enum34 (from pykmip)
  Using cached https://files.pythonhosted.org/packages/bf/3e/31d502c25302814a7c2f1d3959d2a3b3f78e509002ba91aea64993936876/enum34-1.1.6.tar.gz
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/usr/lib64/python3.6/site-packages/setuptools/__init__.py", line 5, in <module>
        import distutils.core
      File "/usr/lib64/python3.6/distutils/core.py", line 16, in <module>
        from distutils.dist import Distribution
      File "/usr/lib64/python3.6/distutils/dist.py", line 9, in <module>
        import re
      File "/usr/lib64/python3.6/re.py", line 142, in <module>
        class RegexFlag(enum.IntFlag):
    AttributeError: module 'enum' has no attribute 'IntFlag'

The "enum-compat" virtual package exists for this very reason. Installs enum34 on older versions of Python, no-op for 3.4+. Please change setup.py and requirements.txt to use this instead.

[PeterHamilton]

@staticglobal Thanks for this. I wasn't aware of enum-compat. I'll update requirements accordingly.

[ctil]

I ran into this issue as well when creating a python 3 environment, so I opened #579.